1. Field of the Invention
The present invention relates generally to implantable devices, and particularly to the construction and assembly of such devices.
2. Related Art
The present invention relates to devices intended to be implanted in the human body, particularly those having internal batteries, or other temperature-sensitive components.
For example, active medical devices typically require a source of electrical power. This may be achieved by passing power transcutaneously via an inductive link, or by providing an implanted power source such as an internal battery. Battery power has the advantage of reducing the need for external components, and also avoids the power losses associated with an RF link. Also, batteries which are rechargeable via an inductive link may provide a degree of independent operation.
A number of precautions are necessary to ensure the highest level of reliability of the electronics in an active medical device. One such precaution is ensuring the electronics are enclosed in a hermetically sealed enclosure with a very low moisture level, typically less than 0.5%. To achieve this low moisture level it is necessary to bake the assembly before final sealing of the enclosure. A typical bake would be 125° C. under vacuum for 16 hours. The final seal is typically achieved by sealing a “bake out hole,” which is a hole in the enclosure, of about 0.15 mm diameter left open to allow the escape of moisture, and which can be sealed by a laser pulse.
While most electrical components can withstand the bake process, batteries often cannot withstand such temperatures. For example, lithium ion battery technology typically has an upper temperature limit of 60° C. When exposed to temperatures, such a battery is irreversibly damaged. Other temperature-sensitive components in implants may similarly be unable to withstand such bake temperatures.